ccc: Introduce ToolChains for mapping Actions to Tools which can

perform them.

 - A ToolChain is a coherent set of tools use in a compilation
   process. The idea is that a ToolChain holds roughly the information
   (specs, search paths, etc.) that is in a single gcc binary.

 - The default ToolChain is selected by the host and will generally
   correspond to what the default system compiler would do. However,
   this can be over-riden for a variety of purposes, for example the
   by the driver driver or for testing.

llvm-svn: 62021